Sterilization with a germicidal agent, such as ethylene oxide gas or ethylene oxide gas mixtures, has played an increasingly important role in sterilizing heat or moisture sensitive materials. Rapid growth in the use of sterile, disposable medical devices is just one consequence of gaseous sterilization with agents such as ethylene oxide. The basic gaseous sterilization process consists of evacuating the sterilization chamber, preconditioning the articles to be sterilized at an optimal relative humidity, generally between 20-70% RH, admitting the sterilizing gas at an appropriate pressure and temperature, maintaining contact between the sterilizing atmosphere and the articles to be sterilized for an appropriate time, and finally discharging and evacuating the chamber to remove the sterilant gas.
Although there are many variations on the basic process, the major factors which have to be controlled in order to effect the sterilization are exposure time, temperature, ethylene oxide pressure or partial pressure, and relative humidity The following prior art references provide a good description of the standard sterilization processes and apparatus with which the sterilizing agents of the invention are useful: "Principles and Methods of Sterilization," pp. 501-530, 2d ed. (1969) by J. J. Perkins; "Ethylene Oxide Gaseous Sterilization," pp 181-208, in Industrial Sterilization International Symposium, 1972 U.S. Pat. No. 3,068,064 and U.S. Pat. No. 3,589,861.
By itself, ethylene oxide is an extremely flammable gas. Its flammability range extends from about 3.5% by volume to 100% by volume in air. Thus, when ethylene oxide is used alone as a sterilizing gas, precautions such as explosion proof equipment are mandatory.
A preferable practice is to blend the ethylene oxide with another inert fluid; diluting the ethylene oxide and rendering the mixture as a whole nonflammable. Two such blends which have been used as sterilizing gases are dichlorodifluoromethane (CFC-12)/ethylene oxide and carbon dioxide/ethylene oxide. These blends are non-azeotropic in nature. As such, they suffer the disadvantage of segregation during vaporization which could lead to potentially flammable or explosive situations if process flow rates, outage volumes, etc. are not closely monitored and controlled.
The CFC-12/ethylene oxide blend is generally supplied as a liquid mixture consisting of 88% by weight CFC-12 and 12% by weight ethylene oxide. This composition is below the critical flammability composition of about 14-15% by weight ethylene oxide in CFC-12, and is therefore nonflammable. A typical hospital sterilization process which utilizes the CFC-12/ethylene oxide blend is performed by evacuating the chamber to about 20-24 inches of mercury vacuum, preconditioning the articles at an optimal relative humidity, and filling the chamber to about 10 psig pressure with the gas mixture. Sterilization is generally performed around 130.degree. F. This procedure provides about 630 milligrams of ethylene oxide per liter. The concentration (mg/liter) of ethylene oxide present in the sterilization chamber is critical in determining the required exposure time and ultimate sterilization efficiency. The Association for the Advancement of Medical Instrumentation (AAMI) recommends an absolute minimum ethylene oxide concentration of 450 mg/liter.
A disadvantage of using CFC-12 in such mixtures is that fully halogenated chlorofluorocarbons such as CFC-12 are implicated in causing environmental problems associated with the earth's protective ozone layer.
Although the major purpose of the inert component in these sterilizing gas mixtures is to mask the flammability characteristics of ethylene oxide, simple substitution of an arbitrary nonflammable fluorocarbon does not necessarily ensure a useful sterilizing gas mixture. First, the flammability properties of the blend must be such that sufficient ethylene oxide (mg/liter at a typical pressure and temperature) is delivered by the blend to effect the sterilization in an appropriate time. If the diluent does not mask the flammability to a sufficient extent, a lower concentration of ethylene oxide must be used to ensure non-flammability. In this event, either a longer time period is required to perform the sterilization, which affects productivity, or greater operating pressures are required to increase the ethylene oxide density in the sterilization chamber. Increasing the operating pressure is generally not a viable option because existing sterilization chambers may not be rated for the increased pressure, and as pointed out by Gunther in U.S. Pat. No. 3,589,861, increased pressure can lead to swelling and rupture of the sealed plastic bags commonly used to package disposable medical devices. Indeed, lower operating pressures are advantageous in this respect.
A candidate inert diluent should preferably also be miscible with ethylene oxide in the liquid phase and should not be too highly volatile that it would segregate from the ethylene oxide to anY great extent during vaporization. Segregation or fractionation can lead to potentially flammable or explosive situations. An azeotrope-like mixture would be useful in this context as it does not fractionate by normal evaporation or distillation processes thereby resulting in the release of the flammable ethylene oxide component.
Accordingly, it is an object of this invention to provide a novel sterilizing gas mixture containing ethylene oxide.
It is an object of this invention to provide such a sterilizing gas mixture which contains an inert fluorocarbon diluent which is considered to be stratospherically safe.
Another object of the invention is to provide a sterilizing gas mixture containing ethylene oxide which is azeotrope-like or non-segregating.
Still another object of the invention is to provide a novel sterilizing gas mixture which incorporates all of the above stated objectives.
Other objects and advantages of the invention will become apparent from the following description of the invention.